1. Technical Field
The present invention relates generally to an automatic transmission and, more particularly, to an automatic transmission controller which modifies a shift schedule when a vehicle is performing under tipout conditions.
2. Discussion
Automatic transmissions automatically adjust the torque and speed which is transmitted to a vehicle's wheels from the vehicle's engine. This adjustment maximizes vehicle performance and fuel efficiency under varying road conditions and vehicle speeds. These transmissions usually contain four or five gears, three or four forward gears and one reverse gear, which provide different gear ratios between the vehicle's wheels and engine. The speed and torque transmitted to the vehicle's wheels are adjusted by shifting the transmission from one gear ratio to another. To change gear ratios, present automatic transmissions contain hydraulic or electric control systems which monitor engine speed, throttle position and a number of other variables which indicate vehicle speed and road conditions. The controller then positions the transmission in the proper gear ratio to maximize vehicle performance and fuel economy under the current road conditions and vehicle speed.
For instance, when a vehicle is traveling at a relatively high and constant velocity, the vehicle throttle position is substantially less than wide open throttle and the engine speed is relatively high. As such, the automatic transmission controller senses this engine speed and throttle position and places the transmission in a high gear ratio, such as third or fourth gear, producing a high speed and low torque at the wheels. When the vehicle is rapidly accelerated, such as when passing another vehicle, the throttle is moved to a substantially open or WOT (wide open throttle) position. Under these conditions, the control senses the changed throttle position and shifts the transmission to a lower gear such as first or second. As a result, more torque is provided to the wheels to allow the vehicle to accelerate. When the throttle resumes its previous position, the controller shifts the transmission back to a high gear. However, under certain conditions, this control system presents certain drawbacks.
Tipout, a high rate of backing out of the throttle, is a condition associated with the rapid transition from an accelerated or WOT condition to a slower or reduced throttle condition. Typically, tipout occurs after a driver passes another vehicle on the road, merges into traffic from an on ramp, or prior to accelerating around a curve. In this situation, the driver places the vehicle in a WOT condition until the vehicle is at a certain position. During WOT, the controller maintains the transmission in a low gear, such as first or second, to provide torque for acceleration. Then, after the driver achieves the required speed to pass the other vehicle or merge into traffic, the driver immediately reduces the throttle position and look around or determine the vehicle position.
Upon reduced throttle position, the controller shifts the transmission back to a high gear such as third or fourth creating the tipout condition. Typically, after the driver looks around, the driver needs the vehicle to once again accelerate to resume normal speed or for other reasons. However, since the vehicle is in a higher gear, it does not have the torque required for proper acceleration. As such, the transmission is, once again, forced to change into a lower gear for acceleration. This behavior is undesirable for a number of reasons. First, response time for accelerating the vehicle is reduced since the transmission must first downshift before the vehicle can accelerate. Second, this behavior requires the transmission to shift a number of times in a short period of time. This increases wear on the transmission components and increases noise, which a vehicle driver finds undesirable. The present invention was developed in light of these drawbacks.